Two different metals have been used extensively in electrolytic capacitors, namely aluminum and tantalum. These electrolytic capacitors are generally used in the filter sections of power supplies and other locations where high capacity is required. Because of their low price, aluminum electrolytics are extensively used. Tantalum capacitors often exhibit higher values of capacity per-unit-volume than aluminum types, but are often more expensive per microfarad. A totally different type of capacitor, called a "double layer capacitor," has even higher capacities per-unit-volume. The wet slug type tantalum capacitor is produced in ratings up to about 1200 microfarads. Double layer capacitors have been produced for commercial uses and these include high capacitance (up to 1,000,000 microfarads or larger) low voltage designs which consist essentially of a pair of paste electrodes and a separator saturated with electrolyte to form a capacitor cell. The past electrodes are from a conductive material, such as both being carbon, as in U.S. Pat. No. 3,536,963, or one of the paste electrodes being of carbon and the other being of a refractory hard boron carbide or the like, as shown in U.S. Pat. No. 3,634,736. Other double layer capacitor cells are disclosed in U.S. Pat. Nos. 3,656,027; 4,014,730; and 4,542,444, hereby incorporated by reference. These devices utilize a carbon paste electrode which is manufactured by mixing finely divided carbon particles with an electrolyte to form a paste and then subsequently forming an electrode from the paste. Activated carbon may be utilized because it has a high surface area to achieve a high farad capacity per-unit-volume of the entire capacitor.
The tantalum wet slug capacitors do not utilize the double layer capacitor cell construction. Instead, they are of essentially a single capacitor construction, with a central anode of porous tantalum and the electrolyte is generally of a sulfuric acid type. The tantalum can becomes the cathode, utilizing not only the base of the can but also the inner surface of the usual cylindrical-shaped can. In this type, the tantalum can is always the cathode and serves an electrochemical purpose, forming a capacitor between the tantalum metal surface and the electrolyte. To this end, the inner tantalum surface is roughened to increase the surface area and often may have a porous layer thereon, such as disclosed in U.S. Pat. No. 4,523,255.
The typical wet slug capacitor has extremely limited reverse voltage capability. Reverse voltage on a typical wet slug tantalum capacitor can disintegrate the usual tantalum oxide layer on the anode, and if any other metal is present, such as silver, this can plate out as dendrites on the anode. If these become long enough, they can touch the closely adjacent cathode, and then when the proper polarity of voltage is applied, these can cause a short-circuit, with consequent high current, rapid heating, and catastrophic failure such as fire or explosion.
The double layer capacitor cell type is considerably different from the wet slug tantalum capacitor. In the former, the capacitor is non-polar, whereas in the wet slug tantalum capacitor, the can is always the cathode. In the double layer capacitor, the container serves no electrochemical purpose; in the wet slug it does serve an electrochemical purpose, being in the path of the flow of electrons into and out of the capacitor. In the double layer capacitor, the container surface need be electronically conductive only at the electrode connection, whereas, in the wet slug the tantalum surface in some cases is electronically insulating, as in the porous cathode configurations. In the double layer, a smooth inner surface is satisfactory, whereas in the wet slug a rough inner surface is desirable in order to increase the surface area and the capacitance. In the double layer, an applied reverse voltage causes no problems, since it is non-polar, taking a charge equally well in either polarity, whereas the wet slug tantalum devices have a very limited reverse voltage range.
The double layer devices also have in the order of 100 to 1000 times more farad capacity per-unit-volume than the wet slug tantalum devices, and hence are even used as an uninterruptible power supply for as much as a week. That is, the technology of the wet slug tantalum capacitors is not applicable to provide the same back-up service as the double layer capacitors.
The presently available double layer capacitors have a number of deficiencies which limit their usefulness, life, and applications, including: a limited operating temperature of -25.degree. C. to +70.degree. C.; corrosion of the container; inability to perform adequately in a high vacuum or high humidity; and limited service life of loss of up to 15% of capacitance when operated for less than two months at 70.degree. C.
There is a considerable need for an extended life, double layer capacitor, especially in military applications, where a device must work properly even though in storage for 20 or 25 years and/or monitored at elevated temperatures. It has been determined that commercial devices over a number of years and under adverse conditions will lose some of the electrolyte. This has two damaging results: the drying out of the paste electrodes to reduce the farad capacity of the unit, and the considerable increase in the equivalent series resistance. This resistance increase is due to a higher concentration and reduction in quantity of the acid electrolyte and internal contact resistance due to corrosion.